A fuel supply system of the fuel injection type in an internal combustion engine for automobiles is generally the like shown in FIG. 13, wherein fuel 2 in a fuel tank 1 is sent out of the fuel tank 1 by a low-pressure pump 3, filtered through a filter 4 and, following receiving pressure adjustment by a low-pressure regulator 5, supplied to a fuel supply apparatus 6, or a high-pressure pump. Fuel is rendered high pressure by the fuel supply apparatus 6 only in the quantity necessary for fuel injection and supplied into common rails 9 of an internal combustion engine (not shown). The remainder of the fuel is released through an electromagnetic valve 17 into the space between a low-pressure damper 12 and a suction valve 13. The quantity necessary for fuel injection is determined by a regulation unit (not shown), which also regulates the electromagnetic valve 17. The high-pressure fuel thus supplied is injected in high-pressure spray into a cylinder (not shown) of an internal combustion engine from a fuel injection valve 10 connected to the common rails 9. A filter 7 and a high-pressure relief valve 8 open in the event of unusual pressures within the common rails 9 (the high-pressure relief valve opening pressures) to prevent the failure of the common rails 9.
The fuel supply apparatus 6, or a high-pressure pump, comprises a filter 11 for filtering the supplied fuel, the low-pressure damper 12 for absorbing the pulses of low-pressure fuel, and a pump 16 that applies pressure to the fuel supplied through the suction valve 13 and discharges the high-pressure fuel through a discharge valve 14 and a fuel pressure holding valve 15. A fuel pressuring room 103 constituted by a plunger 100, a sleeve 101 and a plate 102 of the pump 16 plus the suction valve 13 and the discharge valve 14 is linked to the space between the low-pressure damper 12 and the suction valve 13 via the electromagnetic valve 17. A fuel supply apparatus like the fuel supply apparatus 6 is a variable discharge fuel supply apparatus because the discharge of fuel can be adjusted and is variable via the electromagnetic valve 17, and the electromagnetic valve 17 used in such a fuel supply apparatus is an electromagnetic valve for a variable discharge fuel supply apparatus.
FIG. 14 shows the details of a structure of a conventional variable discharge fuel supply apparatus like this. In FIG. 14, the fuel supply apparatus 6, or a high-pressure pump, comprises integrally a casing 21, the pump 16, or a plunger pump disposed within the casing 21, the electromagnetic valve 17 connected to the fuel pressuring room 103 that is constituted by the plunger 100, the sleeve 101 and the plate 102 of the pump 16 plus the suction valve 13 and the discharge valve 14, and the low-pressure damper 12. The details of the structure of the electromagnetic valve 17 are as shown in FIG. 15. Through the activation of solenoids 22 of the electromagnetic valve 17, the electromagnetic valve 17 opens at the time when fuel is discharged in the quantity required by the regulation unit (not shown) during the discharge process of the pump 16 to release fuel from the fuel pressuring room 103 to the low-pressure space between the low-pressure damper 12 and the suction valve 13 (FIG. 13), thus lowering the pressure within the fuel pressuring room 103 to a level below the pressure in the common rails 9 and opening the discharge valve 14. Once opened, the electromagnetic valve 17 remains so till the pump 16 is shifted to the suction process. It is so designed that the quantity of fuel to be discharged to the common rails 9 can be adjusted through the control of the timing of opening of the electromagnetic valve 17.
In FIG. 15, the electromagnetic valve 17 for a variable discharge fuel supply apparatus comprises an electromagnetic valve body 24 being accommodated within the casing 21 and having a fuel flow path 23 within, a valve seat 25 disposed within the fuel flow path 23 of the electromagnetic valve body 24, a valve 26 for opening or closing the fuel flow path by coming into or out of contact with the valve seat 25 within the electromagnetic valve body 24, and a compression spring 27 for compressing the valve 26 to the valve seat 25.
An electromagnetic valve like the electromagnetic valve 17 for a variable discharge fuel supply apparatus, in which a plunger piston is driven up and down in FIG. 14 by the driving cam disposed coaxially with the unillustrated engine camshaft sucks up and discharges fuel. During the process, through the opening of the electromagnetic valve 17 at the time when a certain quantity of fuel has been discharged into the common rails 9, the high-pressure fuel in the fuel pressuring room 103 is released to the suction side at a lower pressure. Through the control of the timing of opening of the electromagnetic valve 17, the discharge from the fuel supply apparatus is variably adjusted and controlled.
FIG. 16 is a schematic partial sectional view showing the electromagnetic valve 17 in a state in which it is in an open position to the valve seat 25. FIG. 17 is a schematic partial sectional view showing the valve 26 of the electromagnetic valve 17 in a state in which it is in a closed position to the valve seat 25. In the conventional electromagnetic valve 17, a ball seal is used, and it is so constituted that the high-pressure side is in the direction opposing to the spring 27 with the valve seat 25 being taper-shaped and the valve 26 being ball-shaped. The opening area (mm2) relative to the valve lift (mm) in the electromagnetic valve 17 like this is as expressed by the equation shown in FIG. 16, and varies in a way as shown in the graph given in FIG. 18. The strength (N) requisite for the spring to cope with the varying seat diameter (mm) is as expressed by the equation shown in FIG. 17, and varies in a way as shown in the graph given in FIG. 19 (showing the case where the pressure inside the pump is 12 Mpa).
Thus, for the purpose of securing a relief property that is essential to the electromagnetic valve 17 for a variable discharge fuel supply apparatus, it is necessary to increase the distance of the valve lift and extend the seat diameter for a conventional electromagnetic valve 17. When increasing the distance of the valve lift, the fact that the slope representing the valve lift distance-opening area property is dependent on the tapering rate of the tapered seating section must be taken into account (see the equation in FIG. 16). While, when extending the seat diameter, the fact that the slope representing the seat diameter-spring strength property jumps with an increase in the seat diameter must be taken into account (see the equation in FIG. 17).
Thus, when improving the relief property in a conventional apparatus, such problems as the enlargement of the electromagnetic valve 17 and consequential increase in the electric current consumption and lowering of responsiveness that could occur due to the reasons described above must be confronted. Further, the heat generated at the solenoid coils 22 within the electromagnetic valve 17 could increase, and this could induce a shortage between the lines and a malfunction of the electromagnetic valve due to disconnection and the like, and in the worst case, the control of discharge could be put in disorder.
The present invention therefore has as its object the improvement of the relief property and responsiveness, and the provision of an electromagnetic valve for a variable discharge fuel supply apparatus with a small electromagnetic valve, reduced electric current consumption and improved responsiveness.